Field-effect transistors which utilize two-dimensional materials in the transistor channel can be used as versatile detectors of electromagnetic radiation at wavelengths ranging from infrared to ultraviolet. A graphene field-effect transistor (GFET), for example, comprises a semiconducting graphene channel whose conductivity increases when it is illuminated by electromagnetic radiation. This increased conductivity can be measured, and strongly amplified, in a field-effect transistor geometry where the current through the channel is controlled by a gate voltage applied to an electrode adjacent to the channel.
Throughout this disclosure, graphene will be the primary example of a two-dimensional material, and graphene field-effect transistors will be the primary devices. However, examples of other semiconducting two-dimensional materials, which exhibit similar behaviour when illuminated by electromagnetic radiation, will also be given.
It is known that graphene absorbs electromagnetic radiation uniformly in a broad range of wavelengths, but absorption in one graphene layer is limited to a few percentage points of the total radiation intensity at most wavelengths. The optical absorption and spectral responsivity of a graphene field-effect transistor can be increased by preparing an additional photoactive layer adjacent to the graphene channel. Such photoactive layers may be semiconducting.
A built-in electric field (electrostatic potential) may be formed between the photoactive layer and the graphene layer in thermodynamic equilibrium. It may facilitate trapping of minority carriers when electromagnetic radiation illuminates the photoactive material.
Corresponding majority carriers can recirculate in the transistor channel many times before recombining. The photoactive layer may thereby donate charge carriers to the graphene channel when they are released in the photoactive layer by electromagnetic radiation, and change the conductivity of the graphene channel. This carrier multiplication process can greatly improve the sensitivity of a photosensitive field-effect transistor.
Photoactive layers can also expand the spectral response of GFETs to a broader wavelength range. By selecting a semiconductor with a given optical bandgap to the photoactive layer, the photosensitive field-effect transistor can be sensitized to respond particularly strongly to radiation wavelengths which exceed this bandgap.
Document US20150364545 discloses a field-effect transistor with semiconducting layers adjacent to a graphene layer.
A general problem in GFET photodetection assisted by semiconducting photoactive materials is that the magnitude of the multiplicative effect depends on the energetics of the interface between graphene and the photoactive layer. The energetics can be unfavourable if the strength of the built-in electric field is low. This can, for example, be the case when the bandgap of the semiconducting photoactive material corresponds to long infrared wavelengths.